Releasable mill

ABSTRACT

A mill assembly having a milling head which is releasable from the mill body, such as by shifting a ball clutch mechanism. The ball clutch mechanism can be shifted by dropping a pumpable plug through the work string to block fluid flow through the releasable milling head, or by increasing fluid flow through a constriction in the releasable milling head to increase the back pressure above the milling head. A check valve in the milling head can prevent uphole flow through the work string in the event of a pressure excursion. A fishing neck can be attached to the milling head.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 11/804,800, filed May 21, 2007, for “Releasable Mill”, which isa continuation of U.S. patent application Ser. No. 10/916,773, filedAug. 11, 2004, for “Releasable Mill”, which claims the benefit of U.S.Provisional Pat. App. No. 60/495,021, filed Aug. 13, 2003, for“Releasable Bridge Plug Mill”.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is in the field of apparatus used to mill out downholeequipment in a well, such as in an oil or gas well.

2. Background Art

Some oil or gas wells are drilled into locations at which multiple oilor gas formations are found, at different depths. That is, onehydrocarbon formation may be above or below another, and there may bemore than two such formations at different depths. It is common toproduce hydrocarbons from only one selected formation at a time. Onemeans used to assist in this type of production is a plug, which can beinstalled in the bore hole or casing, between two of the formations.Such a plug isolates one formation from another, while allowing accessto the upper formation via the bore hole. It is also common to removesuch a plug, in order to allow access to the lower formation, via thebore hole, for the purpose of producing hydrocarbons, or for otherpurposes.

When such a plug is removed, it is often removed by lowering a mill intothe bore hole or casing, attached to a work string. The mill is usuallyprovided with some type of cutting structure on its lower face, and thiscutting structure is often dressed with some type of cutting material,such as inserts or abrasives. The mill is lowered into contact with theupper end of the plug; then, the work string is rotated, therebyrotating the mill. Alternatively, a downhole motor can be used on thework string, as is commonly known in the art, and the mill can berotated by operating the downhole motor. In either case, as the mill isrotated, the cutting structure cuts the plug into small cuttings, whichare returned to the surface entrained in the drilling fluid which ispumped downhole through the work string. This operation is continueduntil the entire plug is removed, or until a sufficient portion of theplug is removed to allow the remaining portion to fall farther into theborehole.

After this type of operation, it is necessary to remove the mill fromthe bore hole before access to the lower formation is available. This isbecause, although the mill may have passageways for drilling fluid,these fluid passageways are not sufficiently large to provide thedesired degree of access to the lower formation. The mill body itself istypically a substantially solid, comparatively hard, metal body.Therefore, in order to complete the operation, the work string and themill must be pulled from the bore hole to provide the desired access tothe lower formation. As is well known, tripping a work string into orout of a well is a time consuming, expensive process. It is desirable tohave a method and apparatus for removing such plugs, or other types ofobjects in a well bore, while eliminating the necessity for tripping thework string out of the bore hole, to remove the mill and provide accessto the lower formation.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a mill assembly having a releasablemilling head attached to a mill body with a ball clutch mechanism. Themill assembly can be lowered into a bore hole to mill out a plug, afterwhich the milling head can be completely released from the work string,such as by shifting the ball clutch mechanism, and allowed to drop intothe bore hole. Separation of the milling head from the mill body leavesa substantially open bore into and through the work string. The millbody and the work string can be left in the bore hole while productionfrom the lower formation takes place, through this open bore. Themilling head is provided with a check valve in the fluid path, to allowthe downhole flow of drilling fluid during milling, but to prevent theuphole flow of fluids during a kick or pressure excursion. A fishingneck can also be provided on the milling head, to assist in the laterremoval of the milling head where desired.

The novel features of this invention, as well as the invention itself,will be best understood from the attached drawings, taken along with thefollowing description, in which similar reference characters refer tosimilar parts, and in which:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a longitudinal section view of a first embodiment of theapparatus;

FIG. 2 is a lower end view of the milling head portion of the apparatusshown in FIG. 1, and showing the location of the line along which thesection in FIG. 1 is taken;

FIG. 3 is an upper end view of the milling head portion of the apparatusshown in FIG. 1;

FIG. 4 is a lower end view of the mill body portion of the apparatusshown in FIG. 1;

FIG. 5 is a longitudinal section view of the apparatus shown in FIG. 1,after complete separation of the milling head from the mill body;

FIG. 6 is a longitudinal section view of a second embodiment of themilling head of the present invention, with a ball check valve;

FIG. 7 is an expanded longitudinal section view of a third embodiment ofthe apparatus of the present invention, with a flapper check valve and afishing neck;

FIG. 8 is an assembled longitudinal section view of the apparatus shownin FIG. 7;

FIG. 9 is a longitudinal section view of a ball clutch and fishing neckfor use in a fourth embodiment of the apparatus of the presentinvention;

FIG. 10 is a longitudinal section view of a collet for use in the fourthembodiment of the apparatus of the present invention, along with theball clutch and fishing neck shown in FIG. 9; and

FIG. 11 is an assembled longitudinal section view of the fourthembodiment of the apparatus of the present invention, incorporating theball clutch and fishing neck, and the collet, shown in FIGS. 9 and 10.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, the mill assembly apparatus 10 of the presentinvention principally includes a mill body 12, to which a milling head14 is releasably attached, such as by one or more shear screws or pins16. The mill body 12 is adapted to be mounted on a work string (notshown) as is commonly known in the art, such as by threading thereto. Aplurality of cutting inserts 18 can be provided on the lower face 34 ofthe milling head 14 to form a cutting structure. Alternatively, thecutting structure can include milled teeth, crushed carbide, orabrasives, without departing from the spirit of the present invention.

One or more torque lugs 20, better shown in FIG. 3, can be provided onan upwardly facing annular shoulder 46 of the milling head 14. Thesetorque lugs 20 can extend into one or more torque notches 28, bettershown in FIG. 4, formed on the lower end 48 of the mill body 12. Anaxially oriented inner face or shoulder 42 in each torque notch 28 abutsan axially oriented outer face or shoulder 40 on each torque lug 20.Rather than torque lugs and notches, mating shoulders couldalternatively be used. When the milling head 14 is mounted to the millbody 12, the upwardly facing annular shoulder 46 of the milling head 14abuts the lower end 48 of the mill body 12. Also, the upper end 36 ofthe milling head 14 can abut a downwardly facing annular shoulder 38within the mill body 12.

The section shown in FIG. 1 is taken along a broken section line asshown in FIG. 2, to better illustrate a possible placement of the torquelugs 20 and torque notches 28, and the shear pins 16.

A fluid flow path can be provided through the mill body 12 and themilling head 14, which can for example include the inner bore 44 in themill body 12, and a first conical surface 50, a ball seat 30, an innerbore 32, a second conical surface 52, an axial jet 24, and a pluralityof angled jets 26 on the milling head 14. Drilling or milling fluid canbe pumped down the work string (not shown) to flow through this fluidpath in the mill body 12 and the milling head 14, as indicated by thearrows. In addition to the mill assembly apparatus 10, a pumpable ballor plug 22 can be provided for selectively restricting this fluid flow,as will be described below.

The mill assembly apparatus 10, assembled as shown in FIG. 1, is mountedto a work string (not shown) and lowered into a well bore, until thecutting structure on the lower face 34 of the milling head 14 contacts aplug or other item to be milled out of the bore hole. A rotatable workstring or a downhole motor can be used, without departing from thespirit of the present invention. After contacting the plug to be milled,the mill body 12 is rotated in the clockwise direction, as viewed fromthe upper end, rotating the milling head 14 by virtue of the abutment ofthe axially oriented torque shoulders 40 and 42, and causing the inserts18 or other cutting structure to mill the plug away. Cuttings orfragments of the milled plug are removed from the bore hole entrained inthe milling fluid which is pumped through the mill body 12 and themilling head 14 and returned up the annulus to the surface.

After the plug has been milled away, the pumpable plug or ball 22 can bepumped downhole through the work string to land in the ball seat 30 inthe milling head 14. Alternatively, the fluid flow rate can simply beincreased through the apparatus 10 by increasing the speed of the fluidpumps. Either action results in an increased hydraulic pressure at alocation in the fluid flow path as it passes through the milling head.If the pumpable ball 22 is used, the increased hydraulic pressure occursprimarily on the first conical surface 50 and across the top of the ball22. If the increased pump speed is used, the increased hydraulicpressure occurs in the fluid flow path 50, 30, 32, 52, 24, 26. Thisincreased hydraulic pressure exerts an increased downward hydraulicforce on the upwardly facing components of the surfaces of the millinghead 14 which are exposed to the increased pressure. As this downwardhydraulic force reaches a sufficient, predetermined, level, it causesthe shear pins 16 to shear.

When the shear pins 16 shear, the milling head 14 is completely releasedfrom the mill body 12 and completely separates therefrom, as shown inFIG. 5. This complete axial separation of the milling head 14 from themill body 12 allows the milling head 14 to fall downhole, completelyopening up the borehole at the previously plugged location. Since thetorque shoulders 40 and 42 are axially oriented, they are adapted toseparate from each other easily when the shear pins 16 shear, and theydo not interfere with the shearing of the pins 16 or the complete axialseparation of the milling head 14 from the mill body 12.

After complete separation of the milling head 14 from the mill body 12,the inner bore of the mill body 12 is completely open to allow for flowof hydrocarbon fluids upwardly through the mill body 12 as shown by thearrows in FIG. 5. The separated mill body 12 thus performs thereafter assimply an extension of the work string, and the hydrocarbon fluid flowcontinues upwardly through the work string to the surface. Therefore,the complete separation of the milling head 14 from the mill body 12allows for the efficient production of hydrocarbons from the bore hole,through the work string, without pulling and replacing the work stringwith a production tube.

A second embodiment of the milling head is shown in FIG. 6. Thisembodiment of the milling head 140 can be fitted with a check valvecomprising a ball seat 142, a check ball 144, and a spring 146. It canbe seen that, as milling fluid passes downhole through the fluid path inthe milling head 140, the check ball 144 can be lifted off its seat 142,against the bias of the spring 146, to allow flow out the lower end ofthe work string. A kick or pressure excursion sometimes occurs in theformation fluids, which could create an undesirable flow in the upholedirection through the work string. To prevent this, the spring 146biases the check ball 144 toward engagement with its seat 142. Aspressure below the milling head 140 increases above the drilling fluidpressure, this causes the check ball 144 to seat more securely, therebypreventing flow in the uphole direction.

A third embodiment of the apparatus 210 of the present invention isshown in FIGS. 7 and 8. In this embodiment, the mill body 212 is securedto the milling head 214 by shear pins 216 in shear pin bores 224 and 226in the mill body 212 and the milling head 214, respectively. Flowpassages 228 are provided through the milling head 214. However, in thisembodiment, the check valve comprises a swing check type valve, with acheck valve body 262 assembled in the milling head 214, and with aflapper valve 264, which is pivotably mounted to the check valve body262 by a pivot pin 266. The check valve body 262 can be retained in themilling head 214 by one or more snap rings or pins, as is known in theart. The flapper valve 264 is biased toward the closed position by aspring. Flow of fluid down through the apparatus can open the flappervalve 264 against the spring bias, but backflow through the check valveis prevented by shutting of the flapper valve 264, which seats againstthe lower side of the check valve body 262.

Also provided in this embodiment is a fishing neck 260, which isretained in the milling head 214, above the check valve body 262, by oneor more snap rings or pins, as is known in the art. A ball seat 230 isprovided in the upper side of the check valve body 262. When milling hasbeen completed, and it is desired to release the milling head 214 fromthe mill body 212, a ball 222 is pumped downhole through the workstring, to seat in the ball seat 230. Increasing pressure above thepumpable ball 222 then shears the shear pins 216, releasing the millinghead 214 from the mill body 212, as in the first embodiment. If it isdesired to subsequently remove the milling head 214 from the well bore,known fishing techniques can be used to attach to the fishing neck 260and pull the milling head 214.

A fourth embodiment of the apparatus 310 of the present invention isshown in FIGS. 9, 10, and 11. In this embodiment, a ball clutchmechanism 360 is provided, incorporating a fishing neck, and includingone or more ball clutch bores 324 through the wall of the ball clutch360. One or more clutch balls 326 are positioned in the clutch bores324, when the ball clutch 360 is assembled to the mill body 312. Theclutch balls 326 are forced outwardly in an inner annular groove withinthe mill body 312 by a collet 370 which is positioned in the inner boreof the ball clutch 360. The fingers on the upper end of the collet 370are outwardly biased to seat in an inner shoulder of the fishing neckand ball clutch 360. This positioning of the collet 370 releasablyretains the ball clutch 360 to the mill body 312. The ball clutch 360is, in turn, secured to the milling head 314 by one or more snap ringsor pins, as is known in the art. So, the ball clutch mechanism 360releasably retains the milling head 314 to the mill body 312.

Flow passages 328 are provided through the milling head 314. Thisembodiment of the apparatus 310 can be fitted with a check valvecomprising a ball seat 340 in the lower end of the ball clutch 260, acheck ball 344, and a spring 346. It can be seen that, as milling fluidpasses downhole through the fluid path in the milling head 314, thecheck ball 344 can be lifted off its seat 340, against the bias of thespring 346, to allow flow out the lower end of the work string. Toprevent a kick or pressure excursion, the spring 346 biases the checkball 344 toward engagement with its seat 340. As pressure below themilling head 314 increases above the drilling fluid pressure, thiscauses the check ball 344 to seat more securely, thereby preventing flowin the uphole direction.

A ball seat 330 is provided in the upper side of the collet 370. Whenmilling has been completed, and it is desired to release the millinghead 314 from the mill body 312, a ball 322 is pumped downhole throughthe work string, to seat in the ball seat 330. Increasing pressure abovethe pumpable ball 322 then forces the collet fingers inwardly, releasingthe collet 370 from the inner shoulder in the ball clutch 360. After thecollet 370 is released in this fashion, it is forced further downwardlyby fluid pressure. This downward movement of the collet 370 allows theclutch balls 326 to be released from the inner groove in the mill body312, releasing the ball clutch 360 and the milling head 314 from themill body 312. If it is desired to subsequently remove the milling head314 from the well bore, known fishing techniques can be used to attachto the fishing neck on the ball clutch 360 and pull the milling head314.

While the particular invention as herein disclosed is capable ofobtaining the objects hereinbefore stated, it is to be understood thatthis disclosure is merely illustrative of the presently preferredembodiments of the invention.

1. A milling head assembly for use on a work string, comprising: amilling head; at least one cutting structure on said milling head; afluid path through said milling head; and a ball clutch mechanismfixedly attached to one of said milling head or a work string, said ballclutch mechanism being adapted to attach said milling head to a workstring, said ball clutch mechanism being adapted to selectively releasesaid milling head from a work string, thereby completely separating saidmilling head from a work string.
 2. The milling head assembly recited inclaim 1, further comprising a torque transfer device on said millinghead, said torque transfer device being adapted to transfer torque froma work string to said milling head, said torque transfer device beingadapted to allow said separation of said milling head from a workstring.
 3. The milling head assembly recited in claim 1, furthercomprising a flow restricting device, said flow restricting device beingadapted to impede fluid flow through said fluid path at a location onsaid milling head, thereby creating a predetermined hydraulic force onsaid milling head to release said ball clutch mechanism.
 4. The millinghead assembly recited in claim 1, further comprising a check valve insaid fluid path of said milling head, oriented to prevent fluid flow inthe uphole direction through said fluid path.
 5. The milling headassembly recited in claim 4, wherein said check valve is a ball checkvalve.
 6. The milling head assembly recited in claim 4, wherein saidcheck valve is a flapper check valve.
 7. A mill assembly for use on awork string, comprising: a mill body adapted to be attached to a workstring and lowered into a well bore; a milling head; at least onecutting structure on said milling head; a fluid path through said millbody and said milling head; and a ball clutch mechanism fixedly attachedto one of said milling head or said mill body, said ball clutchmechanism being adapted to attach said milling head to said mill body,said ball clutch mechanism being adapted to selectively release saidmilling head from said mill body at a predetermined hydraulic force onsaid milling head, thereby completely separating said milling head fromsaid mill body.
 8. The mill assembly recited in claim 7, furthercomprising a torque transfer device on said milling head, said torquetransfer device being adapted to transfer torque from said mill body tosaid milling head, said torque transfer device being adapted to allowsaid complete separation of said milling head from said mill body. 9.The mill assembly recited in claim 7, further comprising a flowrestricting device, said flow restricting device being adapted to impedefluid flow through said fluid path at a location on said milling head,thereby creating said predetermined hydraulic force on said millinghead.
 10. The mill assembly recited in claim 7, further comprising acheck valve in said fluid path of said milling head, oriented to preventfluid flow in the uphole direction through said fluid path.
 11. Themilling head assembly recited in claim 10, wherein said check valve is aball check valve.
 12. The milling head assembly recited in claim 10,wherein said check valve is a flapper check valve.
 13. A method formilling an object in a well bore, comprising: providing a mill bodyattached to a work string and a milling head mounted to said mill bodywith a ball clutch mechanism, said milling head having a cuttingstructure; lowering said mill body and milling head into a well bore onthe work string; rotating said mill body and milling head to mill anobject in the well bore with said cutting structure; preventing anyfluid flow in the uphole direction through said fluid path; and shiftingsaid ball clutch mechanism to completely release said milling head fromsaid mill body.
 14. The method recited in claim 13, further comprising:providing a separate torque transfer device on said milling head;transferring torque from said mill body to said milling head with saidtorque transfer device; and disengaging said torque transfer device uponsaid complete separation of said milling head from said mill body. 15.The method recited in claim 13, further comprising impeding fluid flowat a location on said milling head, to increase hydraulic pressure andshift said ball clutch mechanism.
 16. The method recited in claim 15,further comprising pumping a plug through said work string with saidfluid flow, to land at said location on said milling head to at leastpartially block said fluid flow, and achieve said hydraulic pressureincrease at said location on said milling head.
 17. The method recitedin claim 15, further comprising: providing a constriction at saidlocation in the path of said fluid flow; and increasing said fluid flowinto said work string to achieve said hydraulic pressure increase atsaid constriction in said fluid flow path.
 18. The method recited inclaim 13, further comprising: providing a fishing neck attached to saidmilling head; and retrieving said milling head from a well bore byattaching to said fishing neck.